Effect of water and ions on the rheological behavior of a low viscosity ammonium-based ionic liquid

The protic ionic liquid trihexylammonium octanoate, [T(6)AC(7)COO], was used to extract copper(II), cobalt(II) and nickel(II) ions from aqueous metal nitrate solutions, in presence or absence of SCN- ions. The ionic liquid and the organic phases were characterized by H-1 NMR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and rheological experiments. The analysis of H-1 NMR chemical shifts of the organic phases shows an up-field shift of the proton of the +NH group of the cation of the ionic liquid, attributable to the existence of hydrogen bonding between this proton and water. The presence of water in the ionic liquid phase increases its viscosity, despite water reduces the interactions between cation and anion of the ionic liquid. This unusual enhancement in viscosity can be associated to the change in its heterogeneous structural organization, as strong interactions between the anion of the ionic liquid and molecules of water. The presence of SCN- anions in the extracted phase increases even more the viscosity of the ionic liquid. This increment can be associated to the interactions between SCN- anions with molecules of water via hydrogen bonding. The rheological measurements in steady state shear show that the ionic liquid and the organic phases exhibit a Newtonian behavior over the range examined, however, the results of the dynamic oscillatory shear experiments show that all the samples exhibit a viscoelastic characteristic, and at low frequencies can be described by the Maxwell model with a single stress relaxation. (C) 2022 Elsevier B.V. All rights reserved.